As shown in FIG. 45, for example, Japanese Laid-Open Patent Publication No. 7-42883 discloses a water pipe 1101 made of a synthetic resin and used as a water supply pipe. The water pipe 1101 comprises an end structure connected to a cylindrical joint 1102 in a lockable manner.
A pipe main body 1101a of the water pipe 1101 has a flange portion 1101b formed at its end and projecting outward from an outer peripheral surface of the pipe main body 1101a along a circumferential direction of the pipe main body 1101a. Connecting cylindrical portions 1103 are formed at respective ends of the joint 1102 (FIG. 45 shows only one of the ends). An inserting cylindrical portion 1103a of each connecting cylindrical portion 1103 extends from the connecting cylindrical portion 1103 along an axial direction of the pipe main body 1101a. 
An annular concave portion 1103b is formed in a part of the connecting cylindrical portion 1103 which is located radially outside the inserting cylindrical portion 1103a. An annular seal member 1107 formed of rubber is installed in the concave portion 1103b. A male thread 1104 is formed around an outer peripheral surface of the connecting cylindrical portion 1103. A female thread 1105a that can be screwed onto the male thread 1104 is formed in an inner peripheral surface of a cover nut 1105 forming the joint 1102.
A sleeve 1106 installed around an outer peripheral surface of the water pipe 1101 is generally cylindrical. The sleeve 1106 has a flange 1106b formed at one end and extending outward along a circumferential direction of the sleeve 1106. In addition, the sleeve 1106 has a tapered reduced diameter portion 1106a formed at the other end.
The joint 1102 is connected to the water pipe 1101 by externally fitting the pipe main body 1101a around the inserting cylindrical portion 1103a until the flange portion 1101b of the pipe main body 1101a abuts against the seal member 1107. With the sleeve 1106 externally fitted around the pipe main body 1101a, the male thread 1105a of the cover nut 1105 is screwed onto the female thread 1104.
As the cover nut 1105 is screwed onto the connecting cylindrical portion 1103, the reduced diameter portion 1106a of the sleeve 1106 decreases owing to the pressure from the inner peripheral surface of the cover nut 1105. Accordingly, an inner peripheral surface of the sleeve 1106 is brought into pressure connection with the outer peripheral surface of the pipe main body 1101a. At the same time, the flange portion 1101b of the pipe main body 1101a is sandwiched between the flange 1106b of the sleeve 1106 and the concave portion 1103b. As a result, the pipe main body 1101a is connected to the connecting cylindrical portion 1103 with the water pipe 1101 locked on the joint 1102.
The flange portion 1101b of the pipe main body 1101a is formed by heating and softening each end of the pipe main body 1101a and pressing the end against a predetermined jig for molding. Thus, the operation of forming the flange portion 10b is very complicated. Further, the flange portion 1101b is formed by molding a synthetic resin. Accordingly, if for example, the flange portion 1101b is repeatedly expanded and contracted by a variation in temperature, it may be deformed. When the flange portion 1101b is deformed, it is unstably sandwiched between the flange 1106b and the concave portion 1103b. Consequently, the pipe main body 1101a and the joint 1102 are unstably connected together.
Japanese Laid-Open Patent Publication No. 2000-2384, shown in FIG. 46 discloses a technique to prevent the leakage of a fluid from the end structure between a joint 2102 and a water pipe 2101 both of which are generally cylindrical. The joint 2102, formed of metal, is connected to a pipe main body 2101a of the water pipe 2101. The joint 2102 has a connecting cylindrical portion 2103 formed at one end and inserted into the water pipe 2101. The joint 2102 has a pipe (not shown) connectibly formed at one end. The connecting cylindrical portion 2103 has a plurality of projections 2103a formed around its outer peripheral surface at fixed intervals and projecting outward along a circumferential direction of the connecting cylindrical portion 2103.
The diameter of each end of the pipe main body 2101a is increased using a jig (not shown) so that when the water pipe 2101 is connected to the joint 2102, the pipe main body 2101a has almost the same inner diameter as that of the connecting cylindrical portion 2103 to reduce the flow loss of a fluid at the connecting cylindrical portion 2103. Before the end of the pipe main body 2101a, the diameter of which has been increased, returns to its original state owing to its own contraction force, the portion of the pipe main body 2101a the diameter of which has been increased is installed around the outer peripheral surface of the connecting cylindrical portion 2103. An inner peripheral surface of the pipe main body 2101a is brought into pressure contact with the outer peripheral surface of the connecting cylindrical portion 2103 because of a decrease in diameter caused by the contraction. Moreover, a fixed ring 2104 formed of a synthetic resin is fitted around the end of the connected pipe main body 2101a. 
The fixed ring 2104 has its diameter reduced by contraction to clamp the end of the pipe main body 2101a from its outer periphery to sandwich the end of the pipe main body 2101a between the fixed ring 2104 and the connecting cylindrical portion 2103. As a result, the projections 2103a cut into the inner peripheral surface of the pipe main body 2101a to connect the water pipe 2101 to the joint 2102 in a lockable manner.
With the water pipe 2101 connected to the joint 2102, the pipe main body 2101a has its diameter reduced and the fixed ring 2104 is clamped to the end of the pipe main body 2101a. Accordingly, the inner peripheral surface of the pipe main body 2101a is brought into pressure contact with the outer peripheral surface of the connecting cylindrical portion 2103. This forms a seal between the water pipe 2101 and the joint 2102. This seal structure prevents the leakage of a fluid from the end of the pipe main body 2101a, thus preventing the leakage of a fluid between the water pipe 2101 and the joint 2102.
If the end of the pipe main body 2101a is installed around the outer peripheral surface of the connecting cylindrical portion 2103, the end must be enlarged so that its inner diameter is larger than the outer diameter of the connecting cylindrical portion 2103. That is, the inner diameter of the end must be increased compared to the state in which the inner peripheral surface of the end of the pipe main body 2101a is in pressure contact with the outer peripheral surface of the connecting cylindrical portion 2103. Thus, an unnecessarily heavy load is imposed on the end of the pipe main body 2101a. The load may damage or deform the end of the pipe main body 2101a. Consequently, while the water pipe 2101 is connected to the joint 2102, a fluid may leak from the end of the pipe main body 2101a. 
Further, to have the diameter of the fixed ring 2104 increased to externally fit the fixed ring 2104 around the end of the pipe main body 2101a, it is necessary to use a tool (not shown) to force the fixed ring 2104 to slide to the end of the pipe main body 2101a the diameter of which has been increased. Thus, apart from the operation of increasing the diameter of the pipe main body 2101a, it is necessary to slide the fixed ring 2104. Therefore, the operation of forming a seal structure at the end of the pipe main body 2101a is very complicated.